The present invention relates to a charging device suitably usable with an image forming apparatus such as a copying machine or a printer, more particularly to a charging device having a charging member contactable to a member to be charged. Conventionally, the use has been made with a non-contact type corona charger as charging means for electrically charging an electrostatic latent image bearing member (image bearing member) such as an electrophotographic photosensitive member, a dielectric member for electrostatic recording or the like in an image forming apparatus of an electrophotographic type, electrostatic recording type or the like.
Recently, a contact charging device in which a charging member supplied with a voltage is contacted to the member to be charged to electrically charge the member to be charged, is used because of the advantages in the low amount of the ozone production, low electric power and so on. Particularly, a roller charging type apparatus using an electroconductive elastic roller charging roller) as the contact charging member, is used because of the advantages in the stabilization of the charging. In such a system, a charging roller is press-contacted to the member to be charged and is rotated while being supplied with a voltage, by which the member to be charged is electrically charged.
Referring first to FIG. 6, there is shown a schematic structure of a charging device of such a roller charging type (a). In this Figure, designated by a reference numeral 3 is a member to be charged, 4 is a charging roller (contact charging member) contacted to the member to be charged 3.
The member to be charged 3 is, for example, an electrophotographic photosensitive member in the form of a rotatable drum as an electrostatic latent image bearing member used in an electrophotographic image forming apparatus, and it is rotated in the clockwise direction as indicated by an arrow at a predetermined peripheral speed.
The charging roller 4 fundamentally comprises an electroconductive core metal 40, an electroconductive elastic layer 41 thereon, which is integral and concentric with the core metal. The charging roller 4 is rotatably supported by bearings to the opposite ends of the core metal, and is urged to the photosensitive member with a predetermined urging force by urging means. Thus, a nip N is formed between the photosensitive member 3 and the elastic layer 41, and the charging roller 4 is rotated by the rotation of the photosensitive member 3.
The charging roller 4 is supplied with a predetermined charging bias from a voltage source S, so that outer surface of the rotating photosensitive drum 3 is electrically charged to a predetermined surface potential.
The electrical charging of the photosensitive member 3 by the charging roller 4 occurs mainly due to the electric discharge from the charging roller 4 to the photosensitive member 3 in small discharge regions a which are fine gaps between the charging roller 4 photosensitive member 3 at lateral and portions of the press-contact nip N. Therefore, the electric charging begins when the voltage applied to the charging roller 4 exceeds a threshold voltage.
For example, when the charging is to be effected to an electrophotographic OPC photosensitive member 3 having a thickness of 15 xcexcm, the charging roller 4 is supplied with a voltage of approx. xe2x88x92560 V, by which the surface potential of the photosensitive member 3 rises, and thereafter, the surface potential of the photosensitive member linearly rises proportionally to the applied voltage (the inclination of rising relative to the applied voltage is 1). The threshold voltage is defined as a charging starting voltage Vth.
Therefore, in order to provide a surface potential of the photosensitive member VD required for an electrophotographic process, the charging roller 4 is to be supplied with a DC voltage of Vth+VD. Here, the contact charging type in which the member to be charged is electrically charged by application of DC voltage alone to the contact charging member, is called xe2x80x9cDC charging typexe2x80x9d.
Since the electrical charging of the photosensitive member 3 by the charging roller 4 occurs mainly due to the electric discharge from the charging roller 4 to the photosensitive member 3 in small discharge regions a which are fine gaps between the charging roller 4 photosensitive member 3 at lateral and portions of the press-contact nip, as described hereinbefore, it is desirable that gaps are uniform in order to accomplish uniform charging. In this sense, the surface of the charging member is smooth.
However, when the charging roller 4 is kept press-contacted to the photosensitive member 3 for a long term, for example, when a process cartridge including a charging roller is left unused for a long term, the charging roller 4 may be deformed permanently at the position where the photosensitive member 3 is contacted thereto.
FIG. 6 illustrates the permanent deformed portion 4a (trace of contact resulting from being left with the press-contact state) at (b) and (c).
When the photosensitive member 1 is electrically charged by the charging roller 4 having such a trace of press-contact 4a, the fine discharge gap is not uniform, and therefore, periodical image density non-uniformity corresponding to the circumferential length of the charging roller 4 results due to the deformation at the portion of trace of press-contact 4a (so-called charging roller trace (C set) ).
In order to prevent the production of the trace of press-contact 4a of the charging roller 4 due to the press-contact to the photosensitive member 3, the charging roller 4 and the photosensitive drum 3 are kept spaced apart when they are left unused. More particularly, the use may be made with (1) a clutch for avoiding the press contacting in the initial unused term (simultaneously with rotation of the photosensitive drum in the printer, the charging roller is brought into contact to the photosensitive drum through the clutch mechanism, (2) a spacing member to be inserted to avoid the press contacting in the initial unused term (prior to the insertion of the process cartridge to the main assembly of the printer, the use the removes the spacing member. However, they lead to complication, cost increase and less usability.
Accordingly, it is a principal object of the present invention to provide a charging device gain which even if the charging roller is the formed due to the conduct between the charging roller and the member to be charged, the occurrence of non-uniform charging can be avoided. It is another object of the present invention to provide a charging device in which the non-uniform charging due to the formation of the charging roller is avoided, with the use of low cost and simple voltage source. It is a further object of the present invention to provide a charging device in which a periodical non-uniform charging due to a deformed portion of a charging roller. It is a further object of the present invention to provide a charging device in which density non-uniformity resulting from the charging non-uniformity is prevented.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.